Electrochemical reactors are devices used to effect the transfer of energy between electricity and matter. These reactors may consist of single electrochemical cells, each with an anode and complimentary cathode, or multiple single cells electrically connected together in series (bipolar mode) or in parallel (monopolar mode). Further, in continuous electrochemical reactors the fluid reactants may be manifolded to flow through multiple cells in series or in parallel.
Electro-chemical processes frequently involve continuous electrochemical reactors using multi-phase reactants that should be intimately contacted in the reactor to obtain high process efficiencies. These processes include gas/liquid (G/L) systems in which reactants are fed to the reactor in the gas and liquid phases, as well as those involving liquid/liquid (L/L) systems, where reactants are fed to the reactor in two immiscible (i.e. mutually insoluble) liquid phases.
A so-called “fuel cell” is a single or multi-cell electrochemical reactor used to convert chemical energy to electricity. With respect to fuel cells, a conventional continuous fuel cell reactor has the fuel and the oxidant fed to the reactor in separate streams that are kept apart in the reactor by separators, typically consisting of an ionically conductive ion selective membrane or porous diaphragm that divides each electrochemical cell into anode and cathode compartments. In such a reactor a multiplicity of single cells may be sized and stacked electrically in series to obtain a desired DC voltage and power output, while the gaseous and/or liquid reactants may flow in series or, preferably, are manifolded in parallel to the cells.
Contrarily, in a continuous mixed-reactant fuel cell (MRFC) reactor there may or may not be separate anode and cathode compartments and the gaseous and/or liquid reactants are in one stream that flows in series or in parallel through the cells. Mixed-reactant fuel cells have potential commercial advantages over conventional fuel cells because they have fewer components and a more simple balance of plant.
In multi-phase electro-synthesis processes (continuous electro-synthesis reactors) the reactants are typically fed to the continuous reactor in separate streams, or via an in-line mixer that disperses one of the phases before entering the reactor. The performance of such processes depends in part on the manner of mixing the feed reactants, which in turn affects the fluid dynamics of the multi-phase flow inside the reactor.
Similarly, for both the conventional and mixed-reactant fuel cell continuous reactors known in the art, the reactants are fed to the reactor in separate conduits (e.g. tubes) or are mixed in a single conduit before entering the reactor. In the case of an MRFC with a liquid reactant, the reactant mixture could potentially be a single phase liquid, a two phase gas/liquid or liquid/liquid or a three-phase combination of gas and two liquids. In the latter two cases (i.e. MRFC 2-phase or 3-phase) the selectivity of electrode reactions and consequent performance of the fuel cell depends in part on the fluid dynamics in the reactor and the uniformity of the reactant mixture. Similarly the performance of a conventional fuel cell with a liquid reactant may depend in part on fluid dynamics in the electrode compartments that affect the contact of the reactant(s) with the solid electrodes.
In the present context, the performance of a continuous electro-synthesis reactor is determined by one or more of the current density (A/m2), selectivity (%) and specific energy consumption (Wh/kg of product). Alternatively, the performance of a fuel cell continuous reactor is measured by one or more of the voltage efficiency (%), Faradaic efficiency (%), energy efficiency (%), superficial power density (W/m2), and the volumetric or gravimetric power density (W/m3 or W/kg). As a rule, “improving” the reactor performance means increasing the values of these performance indicators, with the objective of decreasing the total cost per unit of material or energy output (respectively $/kg and $/J), where the total cost is the sum of the capital and operating costs.